Interferometric distance measuring method for measuring surfaces, and such a measuring arrangement

ABSTRACT

A distance measuring method for measuring surfaces uses a laser source having a frequency that can be modulated to tune a wavelength of a laser beam in a wavelength range. The laser beam is generated with a coherence length to provide a measuring beam and is emitted at the surface, located within a specified distance range, as a measuring beam. The measuring beam is back-scattered by the surface and is received again and used to interferometrically measure the distance from a reference point to the surface. The specified distance range lies at least partly outside of the coherence length. One portion of the laser beam is temporally delayed with respect to another portion, such that the one optical path difference caused by the delay matches the optical path difference that corresponds to a distance in the specified distance range plus or minus the coherence length of the laser.

The invention relates to an interferometric distance measuring methodfor measuring surfaces according to the preamble of claim 1 and such adistance measuring arrangement according to the preamble of claim 8 andalso a measuring device for measuring surfaces.

The requirement exists in many fields of application for measuringsurfaces of objects and therefore also the objects themselves with highprecision. This is true in particular for the manufacturing industry,for which the measuring and checking of surfaces of workpieces has highsignificance. A variety of approaches exist for this purpose, whichextend from contacting methods up to optical sensors. In the field ofhigh-precision optical methods, interferometric measuring principles, inparticular in conjunction with the use of coordinate measuring devices,play an increasing role.

One possibility is the use of white-light interferometry forhigh-precision measuring. In this case, the utilization is eitherscanning, i.e., by adjusting the interferometer, and therefore slowly orwith spectrally resolved detection, typically with restriction to ameasuring range of a few millimeters. The field of use of sucharrangements is therefore restricted and in particular workpieces havinga strongly structured surface and correspondingly varying measuringdistances cannot be measured or can only be measured with severerestrictions, for example, long travel times.

Other methods use a frequency-modulated laser beam as measuringradiation for an interferometric arrangement. Thus, for example, anapproach is known from WO 2009/036861 A1, in which in a method formeasuring surfaces, a frequency-modulated laser beam is generated andemitted onto the surface to be measured. After the measuring radiationreception of the backscattered from the surface as the target, thedistance is determined by interferometry, wherein a measuringinterferometer arm and a reference interferometer arm having a partiallyshared beam path are used. Deviations from the essentially perpendicularincidence of the measuring radiation on the surface in the case ofdistance measurements are taken into consideration by an algorithm oravoided or reduced during the scanning guiding by control of theemission of the measuring radiation.

The partially shared beam path of measuring interferometer arm andreference interferometer arm is delimited in this case by a reflectionwithin the optical measuring head, which thus defines the referenceinterferometer. This so-called common path architecture permits thelocal oscillator plane to be arranged within the measuring head optic,for example, also on the optical exit surface, and therefore close tothe target. The advantage of this construction is that environmentalinfluences, for example, temperature changes or vibrations, act in thesame way on both interferometer arms, so that the generated signals aresubject to the same influences in this regard. However, one disadvantageof the construction is the requirement of a long coherence length, if asufficient signal strength within the operating range is to be ensured.

In contrast, external interferometer arms having adjustable delay areused in the field of white-light interferometry. However, other boundaryconditions are also provided for the common field of application ofmedical technology. Thus, these structures to be scanned or measured arefundamentally different in type and structured less with regard to thedistances. In addition, absolute distance information is not necessaryand the time scales typically required for a measurement are less thanin the case of measurements of industrial parts. As a result of thesurfaces to be measured, longer measuring durations are required hereand as a result of the geometries to be measured, greater measuringranges are typically also necessary. Solutions of this prior art arefound, for example, in US 2004/061865, US 2008/117436, or DE 198 19 762,which describe white-light interferometers, which do not have a tunablelaser source for generating frequency-modulated laser radiation. In U.S.Pat. No. 4,627,731, the division of the light signal upstream of themeasuring interferometer into two paths having modulators of differentfrequencies is used to generate a heterodyne frequency. This so-calledmodulation interferometer also requires precise equalization of the pathlengths.

In the case of interferometric measuring arrangements usingfrequency-modulated laser radiation, however, the measuring range isdelimited by the coherence length thereof, so that the field ofapplication is subject to restrictions and corresponding expenditure isrequired on the control side to be able to scan and measure a measuringobject completely and in a short time.

One problem is to provide an improved measuring method or measuringarrangement, respectively, for measuring surfaces or for detectingsurface topographies.

A further problem is to provide such a measuring method or a measuringarrangement, respectively, which overcomes the restrictions existing dueto the coherence length and therefore increases the measurable distancerange.

These problems are solved or the solutions are refined by the subjectsof claim 1, 8, or 15 or of the dependent claims, respectively.

The invention utilizes an interferometric measuring principle having afrequency-modulated, i.e., tunable laser source and correspondingmeasuring construction, as is also described, for example, in WO2009/036861 A1.

According to the invention, in addition to the normal radiation fieldused for distance measuring, a delayed copy is provided, the delay ofwhich substantially corresponds to the runtime of the actual measuringradiation to the target and back again, so that the effective distanceis reduced. By way of this approach, the operating point of the entiremeasuring arrangement is shifted in the direction toward the target, sothat the measurable maximum distance is enlarged and the restrictionexisting due to the coherence length is overcome. The optical pathdifference caused by the delay corresponds to the optical pathdifference, which corresponds in this case to a distance in thepredefined distance range, plus or minus the coherence length of thelaser. In particular, the optical path difference of the delaycorresponds at most to the optical path difference, which corresponds tothe distance to the surface to be measured, and at least to the opticalpath difference, which corresponds to the distance to the surface to bemeasured, minus the coherence length of the laser or else at least tothe optical path difference, which corresponds to the distance to thesurface to be measured, and at most to the optical path difference,which corresponds to the distance to the surface to be measured, plusthe coherence length of the laser.

One approach for implementing a delayed radiation field according to theinvention is the integration of a beam splitter having downstreamoptical delay section behind the laser source. Such a delay section canbe implemented, for example, both as a free beam optic and also as afiber section in a Mach-Zehnder interferometer. In this way, inprinciple identical radiation fields may be generated, which are onlymutually time-shifted or time-delayed by the optical path difference.

For example, a Mach-Zehnder interferometer can be integrated as a delaysection or delay component in a common path arrangement without changingthe standard design. In this way, for example, the operating point ofthe interferometer can be displaced outside the optic, so that therestrictions normally caused by the coherence length can be overcome. Inthis case, the measuring range of 30 mm, for example, can be maintained,only its length in relation to the optical measuring head is shifted inthe target direction, so that greater distances or spacings to themeasuring object are also implementable. Greater distances in turn allowhigher travel speeds of the sample head and therefore shorter measuringtimes or other measuring path geometries.

A further improvement of the arrangement according to the invention canadditionally be achieved by a focal length of the lens system of thesample head which is designed for the setpoint distance.

To be able to prevent a change of the delay during or between themeasurements, it is advantageous to design the delay component to be asmechanically and thermally robust as possible. The calibration can beperformed in this case by means of known methods, for example, byscanning a reference sphere having known geometry.

A measuring method according to the invention and a measuringarrangement according to the invention for the interferometricmeasurement of surfaces are described or explained in greater detailhereafter on the basis of exemplary embodiments, which are schematicallyillustrated in the drawings solely as examples. In the specific figures:

FIG. 1 shows the schematic illustration of the basic principle accordingto the invention of the optical delay of a radiation fraction in aninterferometric distance measuring arrangement;

FIG. 2 shows the illustration of a first exemplary embodiment of a delaysection for the distance measuring arrangement according to theinvention;

FIG. 3 shows the illustration of the effect of the basic principleaccording to the invention of the optical delay;

FIG. 4 shows the schematic illustration of the interferometric measuringarrangement in a measuring device of the prior art for measuringsurfaces;

FIG. 5 shows the illustration of the structural construction of a samplehead for such a measuring device;

FIG. 6 shows the schematic illustration of transceiver optic for such ameasuring device;

FIG. 7 shows the schematic illustration of a first exemplary embodimentof the interferometric measuring arrangement according to the invention;

FIGS. 8 a-b show the graphic illustration in the frequency domain for afirst example of the radiation fields for the first exemplaryembodiment;

FIGS. 9 a-b show the graphic illustration in the frequency domain for asecond example of the radiation fields for the first exemplaryembodiment;

FIG. 10 shows the schematic illustration of a second exemplaryembodiment of the interferometric measuring arrangement according to theinvention;

FIG. 11 shows the schematic illustration of a third exemplary embodimentof the interferometric measuring arrangement according to the invention;

FIG. 12 shows the illustration of a coherence curve for the followingsimulations;

FIG. 13 shows the illustration of the beam cross section for a firstsimulation example of a distance measuring arrangement of the prior art;

FIG. 14 shows the illustration of a tomogram of the received signal forthe first simulation example;

FIG. 15 shows the illustration of the beam cross section for a secondsimulation example;

FIG. 16 shows the illustration of a tomogram of the received signal forthe second simulation example without delay;

FIG. 17 shows the illustration of the tomogram of the received signalfor the second simulation example with delay according to the invention;

FIG. 18 shows the illustration of the beam cross section for a thirdsimulation example of a distance measuring arrangement according to theinvention having optimized optic;

FIG. 19 shows the illustration of a tomogram of the received signal forthe third simulation example with delay according to the invention; and

FIG. 20 shows the illustration of a second exemplary embodiment of adelay section for the distance measuring arrangement according to theinvention.

FIG. 1 shows the schematic illustration of the basic principle accordingto the invention of the optical delay of a radiation fraction in aninterferometric distance measuring arrangement. In such an arrangementfor measuring industrial workpieces, a laser beam is generated asmeasuring radiation MS by a frequency-modulated, i.e., tunable lasersource 1, wherein it has a coherence length of greater than 1 mm,preferably of greater than 60 mm. In the optical beam path used formeasuring the surface of the workpiece, a delay component isincorporated, which has two optical couplers 2, one of which is designedas a beam splitter for the measuring radiation of thefrequency-modulated laser source 1, wherein this radiation is split intotwo radiation fractions.

One of the two radiation fractions is guided undelayed via the distanceto be measured to the target and back again to the radiation detector,while the other fraction passes through at least one optical delayelement or a delay section 3, by which one of the radiation fractions istime-delayed in relation to the other radiation fraction such that theresulting delay corresponds to twice the run time of the measuringradiation to a distance located outside the coherence length. In theideal case, this distance will correspond to the distance to be measuredto the surface of the workpiece or to another target, but can alsodeviate therefrom. According to the invention, however, the delaysection 3 is designed such that the time delay corresponds to a distancewhich lies within a distance range which at least partially alsocontains possible measuring distances which are greater than thecoherence length. According to the invention, the lower limit of thedistance range can also already lie outside the coherence length.

Therefore, according to the invention, a second radiation field, whichis delayed in relation thereto, is added to the tuned radiation field ofthe prior art. Both radiation fields are superimposed again at theradiation detector, wherein one of them was guided via the delaysection. Instead of the one signal of the arrangement of the prior art,two signals are now generated, which are mutually shifted in accordancewith the delay section and propagate in the measuring interferometer.

In the ideal case, both traversed sections, i.e., optical length of thedelay section and twice the distance to the target, can be identical, sothat a synchronization of the radiation fields on the detector occurs.In the normal case, however, it is sufficient if the delay caused by thedelay section is sufficiently close with respect to time to the delaycaused by the run section to the target and back again. The maximumextent of the difference or the required chronological proximity ispredefined by the measuring range of the arrangement, i.e., themeasuring arrangement can still process the runtime differences oroptical path length differences, which lie within the measuring range,during the measurement. The measuring range is a function of thecoherence length in this case. According to the invention, the measuringrange already existing in arrangements of the prior art is thereforeshifted in the direction toward the target, so that another operatingpoint displaced on the target side results. The maximum extent of theshift is limited here in principle only by the maximum implementabletime delay possibility, i.e., in the normal case, the optical length ofthe delay section. Finally, the delay caused by the target measurementwith respect to the signal running in the reference section of thereference interferometer as a local oscillator is reduced by the delaysection, so that a smaller effective measuring distance results incomparison to the undelayed arrangement. The conditions of the receptionon the radiation detector and therefore the interferometric measuringprinciple used having its restrictions of the measuring range, which arepredefined by the coherence length, are therefore fundamentallymaintained. However, the location of the measuring range is shifted inspace, so that in the case of unchanged coherence length andtarget-related relative relationships of the interferometer, the maximummeasuring distance thereof is changed by the delay section.

FIG. 2 illustrates a first exemplary embodiment of a delay section 3having fixed length for the distance measuring arrangement according tothe invention, wherein the optical delay section 3 is designed inMach-Zehnder configuration. The radiation field generated by the lasersource 1 is guided via a collimator 4 and split by a first polarizingbeam splitter 2′ into two differently polarized radiation fractions,wherein the optical connection between laser source 1, collimator 4, andfirst beam splitter 2′ is preferably embodied in fiber constructionhaving a polarization-obtaining fiber. In this exemplary embodiment inMach-Zehnder configuration, a n-polarized radiation fraction 5 isdirectly relayed, while in contrast the σ-polarized radiation fraction 6is guided via the interferometer having an arm length of approximately10 cm and an inversion prism 8 and finally combined again with the otherradiation fraction.

As a possible design variant, it is advantageous to use a laser source 1which emits in a polarization mode, so that together with the use of apolarization-obtaining fiber as a connection, a coupling at 45° into theinterferometer is possible, which in turn allows a uniform splittinginto the two differently polarized radiation fractions. Alternatively oradditionally, however, a polarization controller connected upstream ofthe delay section 3 can also be used. Both radiation fractions 5 and 6are guided back together in a second polarizing beam splitter 2″ andrelayed via a 45° polarizer and a collimator 4, wherein the connectionscan again also be embodied in fiber construction here. To achievesufficient stability of the interferometer arrangement, the walls 7thereof can be embodied in Zerodur.

The effect of the optical delay unit according to the invention isexplained in FIG. 3, wherein the field strength is illustrated inrelation to the time, i.e., in the time domain. The laser sourcegenerates a radiation field, which is split into two radiationfractions, wherein E(ν,t) designates the undelayed fraction and E(ν,t−τ)designates the radiation fraction delayed by τ=2ΔL/C. In this case, ΔLcorresponds to the length of respectively one of the two arms of theMach-Zehnder interferometer of the delay section and c corresponds tothe speed of light. Both radiation fractions E(ν,t) and E(ν,t−τ) thenpropagate jointly and offset in time through the interferometricmeasuring arrangement.

FIG. 4 shows the schematic illustration of the interferometric measuringarrangement in a measuring device of the prior art for measuringsurfaces, as is known, for example, from WO 2009/036861 A1. Such anarrangement uses a frequency-modulated laser source 1 for generating atleast one laser beam and a radiation detector 11 for receiving themeasuring radiation MS, which is backscattered from a surface 13. Thefrequency-modulated laser source is preferably designed, for example, asa fiber ring laser, such that it has a coherence length of greater than1 mm, in particular in the range from 1 mm to 20 cm, for example, acentral wavelength between 1.3 and 1.55 μm and a tunable wavelengthrange of greater than 40 nm at a dynamic line width of less than 0.02 nmat a coherence length of 60 mm or more. The frequency-modulated lasersource 1 is thus a laser source using which light which is tunable inits wavelength can be emitted within the wavelength range, i.e., lightwhich is frequency-modulated in its light frequency or is tunable in itslight color. The coherence length therefore also permits measurementsover a depth or distance range of several centimeters.

The present invention thus relates to wavelength-tuned interferometry.An interferometric measuring principle using a laser source 1 whichemits in a modulated manner with respect to the wavelength, i.e., withvariable wavelength, is applied, wherein the measurements are performedin the frequency domain. In this case, the laser radiation generated bya laser source 1, for example, a laser diode, is modulated, bytraversing a wavelength ramp and therefore changing the radiation in itsemission frequency, for example.

Such a wavelength ramp can be designed in this case as a classic ramp,i.e., having a sequence of wavelengths to be traversed which rises orfalls substantially linearly. Alternatively, however, the set of thedifferent wavelengths can also be optionally modulated, i.e., in a waydeviating from the linearly arrayed sequence, as long as only the set ofthe wavelengths is acquired and modulated once during one traverse ofthe ramp. The concept of the wavelength ramp therefore comprises in thebroader meaning a set of different wavelengths which can indeed be movedinto a rising or falling sequence, but are not necessarily traversed andmodulated in this sequence. However, a preferred embodiment is designedhaving a sequence of alternating rising and falling linear ramps.

The laser radiation generated by the laser source 1 is coupled via anoptical coupler 10 into the interferometer construction used formeasuring, which is designed in common path geometry, i.e., a partiallyshared interferometer beam path for a measuring interferometer arm and areference interferometer arm. The light, which is modulated in itsfrequency, from the tunable laser source 1, which is applied at theinput of the delay section 3, is thus modulated in its wavelength. Thereference interferometer arm is defined in this case by a reflection atthe optical exit surface of a gradient index lens, so that a constant,in particular known distance is fixed, wherein further back reflectionsare avoided. The reference surface therefore lies in a transceiver optic12, which integrates the components of the transmitter and receiveroptics, within the beam shaping optic used for emitting the laser beam.The measuring interferometer arm is defined, in contrast, by thereflection at the surface 13 to be measured. The back-reflected light ofa measuring interferometer arm and a reference interferometer arm isfinally guided back via the optical coupler 10 onto the beam detector11, which is preferably designed as an InGaAs detector having abandwidth of greater than 100 MHz. Finally, the distance ΔL to bemeasured can be determined in an analysis unit.

In addition, a calibration interferometer (not shown here) having anoptical detector 5 can also be used for taking into consideration orcompensating for nonlinearities in the tuning behavior, wherein thiscalibration interferometer can be embodied in particular in an etalonconfiguration or Mach-Zehnder configuration.

Such a measuring arrangement can be integrated, for example, in a samplehead of a coordinate measuring device for scanning measurement, as isknown, for example, from WO 2009/036861 A1. The structural constructionof such a sample head for such a measuring device is illustrated in FIG.5. The coordinate measuring device has in this case guide means for thedefined scanning guiding of the sample head over the surface to bemeasured and the sample head has at least one emission and receptionbeam path for the emission of measuring radiation MS of theinterferometric distance measuring arrangement.

The sample head is guided by an arm element 14 and a joint 15 as guidemeans in a defined scanning manner over the surface to be measured,wherein a rotation of the joint 15 with respect to the arm element 14 isalso possible. By way of the rotational ability in relation to the armelement 14 and the downstream joint 15, the sample head can well followangled or strongly varying surface profiles. Fundamentally, however,still further rotational or translational degrees of freedom can beintegrated in the guide means, to allow a further improved guiding ofthe sample head.

The sample head has at least one surface-side emission and receptionbeam path of the measuring beam MS. In this embodiment, the beam pathsare guided through a thin tube, which contains the transceiver optic 12.The radiation detector itself or optical waveguides for relaying to aradiation detector integrated at another location can already bearranged in the thicker part 16 adjoining this tube. The sample head iscontrolled by the guide means such that the condition of substantiallyperpendicular incidence of the laser beam on the surface is maintained,in particular a deviation of +/−5° to the surface normal is notexceeded. The sample head can in this case be moved such that it ismoved continuously having constant alignment relative to the surfacetangent, in particular having emission and reception beam path orientedperpendicularly to the surface tangent.

FIG. 6 schematically shows the integration of transceiver optic 12 intothe tube of the sample head. In this design, a fiber 12 a is used forguiding the measuring radiation MS to be emitted and also to bereflected. The emission is performed in this case through a gradientindex lens 12 b arranged in the tubular part, which emits the measuringradiation onto the surface 13 to be measured and couples the measuringradiation MS reflected therefrom back into the fiber 12 a.

FIG. 7 shows the integration of a delay section into the arrangementfrom FIG. 4 to implement a first exemplary embodiment of theinterferometric measuring arrangement according to the invention. Behindthe laser source 1 and a first beam splitter 2 for the measuringradiation, which splits it into two radiation fractions, at least oneoptical delay section 3 is incorporated, by which one of the radiationfractions can be time-delayed in relation to the other radiationfraction such that the delay which can be generated corresponds to twicethe run time of the measuring radiation MS to a distance lying outsidethe coherence length, wherein this distance d to be measured cancorrespond to the surface of the target 13. By way of the use accordingto the invention of the delay section 3, measurements can now also becarried out to targets, the distance of which to the measuringarrangement is outside the boundaries set by the coherence length, butin particular is greater than the coherence length.

In this first exemplary embodiment, the delay section 3 is arranged inthe beam path before the transceiver optic 12, so that the delay occursbefore the emission.

This first exemplary embodiment can also have in the interferometricdistance measuring arrangement a further interferometer as a calibrationinterferometer, wherein this can also be embodied in etalonconfiguration or Mach-Zehnder configuration.

FIGS. 8 a-b and FIGS. 9 a-b illustrate the radiation fields in thefrequency domain for a first example and a second example of the lengthof the delay section in the arrangement of the first exemplaryembodiment.

FIGS. 8 a-b show the graphic representation in the frequency domain fora first example, wherein by way of the use of a measuring interferometerarm and a reference interferometer arm and by way of the two radiationfractions, a total of four radiation fields are generated, which aresuperimposed during the interferometric distance measurements on theradiation detector.

The reflections of the reference arm as a local oscillator and of thetarget in the measuring arm are delayed in relation to one another bythe runtime via the target distance d. Since two radiation fractions arecoupled into the interferometric measuring arrangement having thereference arm and the measuring arm, a total of four interferingradiation fields therefore result on the radiation detector, wherein inthe figures, the undelayed radiation fields are indicated with 1 and thedelayed radiation fields are indicated with 2 and also L is indicatedfor the local oscillator (reference arm) and T is indicated for thetarget (measuring arm). As a result of the frequency modulation of thelaser radiation, a time interval or a run section difference alsocorresponds in this case to a spectral difference δf.

The detected intensity I is a product of the radiation fields in thetime or frequency domain, the Fourier transformation FT of the intensityI is a folding of the Fourier transformation FT of the fields E.

I=|E| ² =E(t)· E(t)

FT(I)=FT(E)

FT(E)

where

E(t)=E _(L1)(t)+E _(L2)(t−τ _(MZ))+E _(T1)(t−τ _(T))+E _(T2)(t−τ_(T)−τ_(MZ))

In this case, E_(L1)(t) designates the undelayed radiation fractionwhich only runs via the reference section, E_(L2)(t−τ_(MZ)) designatesthe delayed radiation fraction which only runs via the referencesection, E_(T1)(t−τ_(T)) designates the undelayed radiation fractionwhich runs via the target distance, and finally E_(T2)(t−τ_(T)−τ_(MZ))designates the radiation fraction which is both delayed and also runsvia the target distance. In this case, τ_(MZ) represents the runtime ofthe radiation fractions guided via the delay section, and τ_(T)represents the runtime of the radiation fractions which run via thetarget distance. The spectral intervals δf(2 L) and δf(2 d) can berepresented as functions of the optical path differences L and d.

In the graphic representation in the frequency domain of FIG. 8 a,therefore four frequencies of the four radiation fractions or radiationfields result, from which the four beat frequencies shown in FIG. 8 bresult after the folding. The delayed radiation field interferes withthe undelayed field in this case, so that an enlargement of the targetdistance or of the corresponding spectral interval δf(2d), indicated bythe arrow in FIG. 8 a, results in a change of the frequency e(f_(T1),t)and therefore the shift shown in FIG. 8 b of the beat frequency i_(L2)_(—) _(T1).

For a frequency modulation with an increase of the optical frequency f,the first undelayed reflection has the highest frequency e(f_(L1),t) atthe time t. After the folding, the interference term of the two L fieldshas the highest intensity, but is suppressed because of its locationoutside the coherence range. The lowest beat frequency component i_(L2)_(—) _(T1) of the interference of the radiation fields e(f_(T1),t) ande(f_(L2),t), in contrast, represents the desired useful signal.

FIGS. 9 a-b show the graphic representation in the frequency domain fora second example of the radiation fields for the first exemplaryembodiment, in which the delay caused by the delay component is selectedto be greater than the runtime via the target section or is greater thantwice the runtime of the measuring radiation to the surface to bemeasured.

In this case, the enlargement shown in FIG. 9 a of the target distance dor of the corresponding spectral interval δf(2d) results in a reductionof the beat frequency for i_(L2) _(—) _(T1) in FIG. 9 b. Such a shiftcan offer advantages, since in this case the beats i_(L1-T2) and i_(T1)_(—) _(T2)+i_(L1) _(—) _(L2) are also shifted toward higher frequenciesand therefore can be suppressed more strongly because of coherence.

In general, both sides of the coherence length with the exception of adirect-current region become usable by shifting the operating rangeoutside the normal coherence length. However, the unambiguity is lostand care must be taken so that the correct side of the operatingdistance is selected.

FIG. 10 shows the schematic illustration of a second exemplaryembodiment of the interferometric measuring arrangement according to theinvention, in which the delay section 3 is incorporated in the beam pathafter the integrated transceiver optic 12 in an arrangement according toFIG. 4, so that the splitting into the radiation fractions with delay ofone of the parts only occurs after the reception and immediately beforethe radiation detector 11. According to the invention, the entire delaycan also be caused by different partial delay sections, however, whichcan also be arranged at various points of the beam path, if the desiredtotal delay or optical path difference results for a radiation fraction.

A third exemplary embodiment of an interferometric measuring arrangementaccording to the invention is schematically illustrated in FIG. 11.While in the first and second exemplary embodiments, fixed delaysections of a defined length are used, in this case, with a constructionotherwise unchanged from the first exemplary embodiment according toFIG. 7, a number of delay sections of different length which can beswitched over is used, so that a plurality of selectable discrete delaytimes is provided. A certain overlap between the delay sections inconjunction with the possible coherence length can be advantageous inthe calibration of the individual delay sections. In this case, thelength difference between the delay sections is somewhat less than thecoherence length, whereby a measuring range overlap occurs. Upon leavingone range, one also reaches the next range by switching over to the nextdelay length. The same distance can then be measured using two delaysections, which permits an assumption of a distance calibration. Thedelay sections can be formed in this case as fibers, as are available asstandard components for applications of optical coherence tomography.Thus, for example, the producers General Photonics, Newport, OZoptics,and Santec offer as standard products fiber-coupled optical delaysections having delay times of up to 350 ps or a length of 110 mm.

Alternatively to delay sections which can be switched over, according tothe invention, continuously or discreetly adjustable variants of delaysections, for example, interferometers having adjustable arm lengths,can also be used.

The effect of a delay section on measurements is illustrated in thefollowing FIGS. 12 to 19 on the basis of simple simulation results. Inthis case, FIG. 12 shows the illustration of a coherence curve for thefollowing simulations, which illustrate signal strengths and coherenceeffects. The fundamental coherence curve is defined as follows:

${{coh}(z)} = ^{- {(\frac{2\; {z \cdot \sqrt{\ln {(2)}}}}{L_{coh}})}^{2}}$

with z as an optical path difference and a coherence length ofL_(coh):=50 mm.

The following definitions and equations apply for the simulations:

time delay:

${\tau (d)}:=\frac{2\; d}{c}$

phase: φ(t,τ):=π·ν(t−τ)·(t−τ)laser amplitude: E_(Laser) _(—) ₀:=1laser field: E_(Laser)(t,d):=E_(Laser) _(—) ₀·e^(i·φ(t,τ(d)))local oscillator

-   -   length: d_(L):=0 m    -   reflectivity: R:=1%    -   oscillator-laser field: E_(L)(t):=E_(Laser)(t,d_(L))·√{square        root over (R)}        target-laser field: E_(T)(t):=E_(Laser)(t,d_(T))·√{square root        over (R·L)}        radiation detector field: E:=(E_(L)(t)+E_(T)(t))

FIG. 13 shows the beam cross section for a first simulation example of adistance measuring arrangement of the prior art without delay section.

The parameters for this example having a target distance of 30 mm and anoptical path difference of 60 mm resulting therefrom read as follows:

Beam waist: w₀:=120 μmdistance up to the beam waist: d_(w) ₀ :=25 mm

numeric aperture:

${NA}:={\frac{w_{0}}{z_{0}\left( w_{0} \right)} = 0.00411}$

exit pupil: D:=w(0,w₀,d_(w) ₀ )·2=0.316 mmRayleigh length: z₀(w₀)=29.2 mmtarget loss: RL:=NA²·Albedoand a power level, resulting from the target loss, of −58 dBm, whereinthe albedo of a dark metal surface assumed as a target is set at 10%.Distance z (horizontal) and beam cross section (vertical) are eachspecified in millimeters.

For the sake of simplicity, to illustrate the coherence influence, it isapplied as a modulation loss in the Fourier space, i.e., the Fouriertransformed P′=FT(p) of the detected power p=(E·Ē) is multiplied by thecoherence function P=P′·coh. The distance d or the optical pathdifference (OPD_(T)≈2·d in air) corresponds to the frequency f via theequation

${OPD}_{T} = \frac{c \cdot f}{\gamma}$

with the spectral tuning rate y of the laser of

$\gamma = {20{\frac{THZ}{ms}.}}$

The associated tomogram of the received signal for the first simulationexample is illustrated in FIG. 14, wherein the power level in dB isplotted against the optical path difference d specified in millimeters.In reality, from a distance of 30 mm, the detection limit is reachedbecause of coherence losses and noises. Therefore, in the simulationwhich does not consider these influences, the required signal strengthshould lie above the threshold, which is illustrated by a dashed line,of −60 dB, so that the conditions prevailing in reality can be takeninto consideration.

As can be seen in the figure, the signal strength, shown by solid lines,without delay according to the invention reaches its maximum at anoptical path difference of 60 mm or at a target distance of 30 mm and istherefore slightly above the sensitivity of approximately −60 dB.

FIG. 15 shows the illustration of the beam cross section for a secondsimulation example. The target distance is now 10 cm, so that an opticalpath difference (OPD) of 200 mm results. The numeric aperturecorresponds to the first simulation example from FIG. 13. The distanceto the beam waist is 100 mm, so that the exit pupil has a diameter of0.857 mm.

As can be seen from the associated tomogram of the received signalillustrated in FIG. 16, the maximum of the signal strength issignificantly below the threshold of −60 dB to be set for realisticconditions, and is therefore for below a level of detectability.

This is contrasted with the results of a simulation having introductionaccording to the invention of a delay, as illustrated in FIG. 17 for thesecond simulation example.

A possible range is predefined by the coherence length of the laser inthe case of the selection of the optical path difference of the delaysection OPD_(MZ). For a good signal analysis without delay, the distanceor the OPD_(T) thereof should be in the range of the coherence length:

0<OPD_(T) <L _(coh)

If a delay section is used, this range is shifted by the OPD_(MZ) ofthis section:

OPD_(MZ)<OPD_(T) <L _(coh)+OPD_(MZ)

The minimal delay section is in this case

OPD_(MZmin)=OPD_(T) −L _(coh),

and the maximum is

OPD_(MZmax)=OPD_(T),

so that for the selection of the length of the delay section, thepossible range results from

OPD_(MZmin)<OPD_(MZ)<OPD_(MZmax)

OPD_(T) −L _(coh)<OPD_(MZ)<OPD_(T).

In this case, which is also described by FIG. 8, an enlargement of thetarget distance results in an enlargement of the measured beatfrequency, which corresponds to the so-called “normal” measuring range.The utilization of the other range, corresponding to FIG. 9, is alsopossible, in which the delay section is greater than the OPD of thedistance and the enlargement of the target distance results in areduction of the beat frequency, the so-called “inverse” measuringrange:

OPD_(T)<OPD_(MZ)<OPD_(T) +L _(coh)

If the unambiguity of the measuring range—normal or inverse—can bedetermined by a movement of the target, for example, the delay distancecan be in both ranges:

OPD_(T) −L _(coh)<OPD_(MZ)<OPD_(T) +L _(coh)

In the above-mentioned example with

d=100 mm→OPD_(T)=200 mm and L _(coh)=50 mm,

and the restriction to the normal measuring range, the delay sectionmust be in the range

150 mm<OPD_(MZ)<200 mm.

If a delay section is used, the laser field consists of two terms,wherein they are mutually delayed and d_(MZ) designates the length ofthe delay section according to the invention (in air, the equation

$d_{MZ} \approx \frac{{OPD}_{MZ}}{2}$

applies)

${E_{Laser}\left( {t,d} \right)}:={\frac{1}{4}{E_{{Laser\_}0} \cdot ^{ \cdot {\varphi {({t,{\tau {(d)}}})}}} \cdot \frac{1}{4}}{E_{{Laser\_}0} \cdot ^{ \cdot {\varphi {({t,{\tau {({d + d_{MZ}})}}})}}}}}$

The losses generated by the splitting and guiding together of theradiation field are taken into consideration by the factor ¼. The targetdistance is again 10 cm and therefore the optical path difference(OPD_(T)) is 200 mm. To cause a delay, an additional section of thelength of, for example, d_(MZ)=90 mm (OPD=180 mm) is introduced for oneof the two radiation fractions, which lies within the possible normalrange.

In the tomogram, the effects of three of the four resulting beatfrequencies are now identifiable. At 200 mm, the normal signalassignable to the target is recognizable, while in contrast at 180 mm,the signal associated with the delay section occurs. At 20 mm, theinterference signal of delayed local oscillator radiation field andundelayed measuring interferometer radiation field, i.e., theinterferometer arm comprising the target, is recognizable.

The interference signal located at 380 mm, composed of delayed localoscillator radiation field and delayed measuring interferometerradiation field, is not shown in the figure for reasons of clarity.

The signal at 20 mm is, in spite of the additional losses of 6 dB causedby the delay section, still above the sensitivity threshold andtherefore well detectable.

FIG. 18 shows the illustration of the beam cross section for a thirdsimulation example of a distance measuring arrangement according to theinvention having an optic optimized for a target distance of 100 mm. Thefollowing parameters apply for this simulation example:

beam waist: w₀:=25 μmdistance up to the beam waist: d_(w) ₀ :=100 mmnumeric aperture:

${NA}:={\frac{w_{0}}{z_{0}\left( w_{0} \right)} = 0.01974}$

Rayleigh length: z₀(w₀)=1.267 mmexit pupil D:=w(0,w₀,d_(w) ₀ )·2=3.947 mm

The albedo of a dark metal surface assumed as a target is set in thisexample at 10%, so that a power level resulting from the target loss of−44 dBm results. The numeric aperture can therefore be enlarged by thefactor 5 and the losses can be reduced by 7 dBm, which means acorrespondingly higher signal strength.

The associated tomogram of the received signal is illustrated in FIG. 19for the third simulation example with delay according to the invention.The significant exceeding of the detectability threshold at 20 mm can berecognized clearly.

FIG. 20 shows a concrete embodiment of the delay section 3 illustratedin FIG. 2. The coupling is performed from below into a first 50% beamsplitter 2′″. A first radiation part 6 propagates in drilled-out andsealed air channels in a Zerodur part 7. A second radiation fraction 5to a second 50% beam splitter 2″″, which guides together both radiationfractions 5 and 6 again. The two radiation fractions 2′″ and 2″″ can bemanufactured from SiO₂, for example, and the coupling and decouplingsurfaces thereof can have an antireflective coating. The inversion prism8 from FIG. 2 is embodied with the two illustrated reflectively coated(for example, with gold) Zerodur parts 8′ and 8″.

The density in the air channels does not change via temperature and theoptical path remains constant. The paths within the beam splitter areidentical for both interferometer arms. The illustrated part 3 of theMach-Zehnder interferometer is therefore athermal.

1-15. (canceled)
 16. A distance measuring method for measuring surfaces,comprising: generating a laser beam, the wavelength of which is tunableby a frequency modulation of a laser source in a wavelength range toprovide measuring radiation having a coherence length; emitting themeasuring radiation onto the surface, which is located within apredefined distance range; receiving the measuring radiationbackscattered from the surface; and interferometric distance measuringfrom a reference point to the surface employing a measuringinterferometer arm and a reference interferometer arm; wherein: thepredefined distance range lies at least partially outside the coherencelength; and the measuring radiation is split into two radiationfractions wherein one of the radiation fractions is time-delayed using adelay section in relation to the other fraction such that an opticalpath difference thus caused corresponds to an optical path difference,which corresponds to a distance in the predefined distance range, plusor minus the coherence length of the laser, and wherein the delaysection is at least partially embodied in Zerodur.
 17. The distancemeasuring method as claims in claim 16, wherein: the measuring radiationis emitted and is received again during a scanning guiding over thesurface to be measured; and during the interferometric distancemeasurements, the measuring interferometer arm and referenceinterferometer arm have a partially shared beam path having a referencesurface, which defines the reference interferometer arm and lies withina beam shaping optic used for emitting the laser beam.
 18. The distancemeasuring method as claimed in claim 16, wherein the coherence length isgreater than 1 mm.
 19. The distance measuring method as claimed in claim16, further comprising: splitting the measuring radiation into tworadiation fractions, as a first radiation fraction and a secondradiation fraction, wherein the first radiation fraction propagates insealed air channels in a Zerodur part; and combining the first radiationfraction and the second radiation fraction.
 20. The distance measuringmethod as claimed in claim 16, wherein: the optical path differencecaused by the delay corresponds: at most to the optical path difference,which corresponds to the distance to the surface to be measured; and atleast to the optical path difference, which corresponds to the distanceto the surface to be measured, minus the coherence length of the laser;or the optical path difference caused by the delay corresponds: at leastto the optical path difference, which corresponds to the distance to thesurface to be measured; and at most to the optical path difference,which corresponds to the distance to the surface to be measured, plusthe coherence length of the laser.
 21. The distance measuring method asclaimed in claim 16, wherein: the delay occurs before the emission ofthe measuring radiation onto the surface to be measured.
 22. Thedistance measuring method as claimed in claim 16, wherein: the delayoccurs after the reception of the measuring radiation backscattered fromthe surface to be measured.
 23. The distance measuring method as claimedin claim 16, wherein: a plurality of selectable discrete delay times isprovided.
 24. An interferometric distance measuring arrangement formeasuring surfaces, comprising: a frequency-modulated laser source forgenerating at least one laser beam, the wavelength of which is tunableby the frequency-modulated laser source, in a wavelength range, forproviding measuring radiation having a coherence length; an optical beampath having: a transmitting optic for emitting the measuring radiationonto the surface; a receiving optic for receiving the measuringradiation backscattered from the surface; and a measuring interferometerarm and a reference interferometer arm; a radiation detector forreceiving the measuring radiation backscattered from the surface; andone analysis unit for determining the distance from a reference point ofthe distance measuring arrangement to the surface, wherein: at least onebeam splitter for the measuring radiation, which splits this measuringradiation into two radiation fractions; at least one optical delaysection, by which one of the radiation fractions can be time-delayed inrelation to the other radiation fraction such that an optical pathdifference thus caused corresponds to an optical path difference, whichcorresponds to a distance in the predefined distance range, plus orminus the coherence length of the laser; and the delay section is formedat least partially in Zerodur.
 25. The distance measuring arrangement asclaimed in claim 24, wherein: the radiation detector comprises an InGaAsdetector having a bandwidth of greater than 100 MHz
 26. The distancemeasuring arrangement as claimed in claim 24, wherein: the optical delaysection is designed in Mach-Zehnder configuration having opticalconnections in fiber construction; after a first 50% beam splitter, afirst radiation fraction propagates in drilled-out and sealed airchannels in a Zerodur part and, using a second 50% beam splitter, asecond radiation fraction, branched off by the first 50% beam splitter,is guided together with the first radiation fraction; the delay sectionincludes an athermal interferometer arrangement such that the opticalpath in the air channels remains substantially constant in the event oftemperature changes; and the paths within first and second beamsplitters for both arms of the interferometer arrangement of the delaysection are equal.
 27. The distance measuring arrangement as claimed inclaim 26, wherein: an inversion prism is embodied on the Zerodur partusing two reflectively coated Zerodur parts; and/or the first beamsplitter and second beam splitter are manufactured from SiO2, whereinthe coupling and decoupling surfaces thereof have antireflectivecoatings.
 28. The distance measuring arrangement as claimed in claim 24,wherein: the beam splitter splits the measuring radiation into twodifferently polarized radiation fractions.
 29. The distance measuringarrangement as claimed in claim 24, wherein the delay section: iscontinuously adjustable; or is selectable from a plurality of delaysections of different lengths, wherein the delay sections are formed byoptical fibers of different lengths.
 30. The distance measuringarrangement as claimed in claim 24, wherein: the delay section isarranged in the beam path before the transmitting optic.
 31. Thedistance measuring arrangement as claimed in claim 24, wherein: thedelay section is arranged in the beam path after the receiving optic.32. The distance measuring arrangement as claimed in claim 24, wherein:the interferometric distance measuring arrangement has a furtherinterferometer as a calibration interferometer in etalon configurationor Mach-Zehnder configuration.
 33. The distance measuring arrangement asclaimed in claim 24, wherein: the receiving optic includes a combinedtransmitting and receiving optic.
 34. The distance measuring arrangementas claimed in claim 24, wherein: the measuring interferometer arm andthe reference interferometer arm have a partially shared beam path. 35.A coordinate measuring device for measuring industrial workpieces,comprising: guide means for the defined scanning guiding of a samplehead over the surface to be measured; and an interferometric distancemeasuring arrangement as claimed in claim 24, wherein: the sample headhas at least one emission and reception beam path for the emission ofthe measuring radiation and the beam path in the measuringinterferometer arm and reference interferometer arm have a partiallyshared part, which defines the reference interferometer arm and lieswithin the beam shaping optic used for emitting the laser beam, thereference interferometer arm being defined by a reflection at theoptical exit surface of a gradient index lens of the beam shaping optic.